1. Field of the Invention
This invention relates to the transmission of a sequence of high quality images for display on a visual display unit. It has particular application in the field of telepathology where magnified images obtained by scanning a medical specimen on the stage of an optical microscope are transmitted from a local pathologist to a remote consultant for diagnosis.
2. Discussion of the Background
For general diagnostic practice in tumour histopathology and cytopathology, because of the potentially serious consequences of misdiagnosis, it is hardly ever acceptable to examine just one or a few static images from the microscope, no matter how high their individual quality may be. Instead, it is accepted that the diagnositician must be free to examine any part of the specimen, at any of the magnification factors which the microscope allows. Thus the remote consultant should have the ability to `scan` the field of view across the specimen on the microscope stage; both along the left-right (-x) axis and the top-bottom (-y) axis of the field of view. The consultant should also be able to move the stage in the longitudinal (-z) axis of the microscope to adjust the focus.
Controlling the microscope stage through verbal instructions to a local pathologist is unacceptably slow and unreliable. Transmitting a complete set of images which together cover the whole specimen could be done automatically using a motorised stage and suitable camera control software, but would require the transmission of around 4000 images for a histopathology section of 15 mm by 10 mm. This again is unacceptable.
Accordingly, remote control of the sender's microscope is virtually essential for a practical system.
One such system is described, for example, in U.S. Pat. Nos. 5,216,596 and 5,297,034. In this known system, the magnified image of the specimen is recorded by a video camera and converted to an electronic video signal which is then transmitted over a communication link to a remote video display monitor. Control signals are generated by a computer processing unit at the remote workstation for remotely controlling the functions of the microscope, including motorised stage movement, magnification, focus and illumination control.
The main problem with this known system is that the quality of the image viewed by the remote consultant on the display monitor is well below the quality that would be seen by viewing the specimen directly through the microscope.
There is an emerging consensus that 1024&gt;768 is the minimum acceptable pixel format for display of diagnostic-quality images on a high-resolution colour monitor. This rules out the use of all analog video cameras, monitors and image compression/decompression devices (codecs) which are based on broadcast standards such as PAL or NTSC. The bandwidth limitation imposed by the broadcase standards reduces the effective pixel number in each image to about a quarter of the number required for diagnostic resolution; in addition, in composite TV equipment, the colour resolution of the signal is further reduced by the chrominance subsampling.
Although digital videocameras with CCD chips are now available which are suitable for capturing high-resolution microscope images, these cannot be used to directly display images on composite video monitors. Instead they are designed to work with image digitisers (frame grabbers) by means of which a digital representation of the image is stored in RAM, or in storage medium such as magnetic disc or CD ROM. To visually display such an image, it must be written to an `RGB` colour monitor with a display driver capable of handling images of at least 1024.times.768 pixels at 8 or more bits/colour channel.
The overriding problem in the use of digital cameras in a telepathology diagnostic system is that, unlike analog video cameras, digital cameras cannot provide images at video rate: in fact a maximum rgb frame rate of about 1/25 of video rate (2/sec), is typical, and in several cases the frame rate is less than 1/sec.
Although standard analog TV videocodec technology can be used to compress the video images such that the required bandwidth is reduced to a practical level (say 384 kbit/sec), if 50 images must be transmitted per second, this allows 384/50 or about 8 kbits per image. Since the final image is normally reduced to a size of (512.times.384) 24 bit pixels (or about 4 Mbit), this requires the codec to perform (lossy) image compression of 500:1. This can only be achieved at the expense of significant image degradation.
To achieve smooth scrolling of the field of view of a digital videocamera across a specimen, the image refresh rate in the viewport must be comparable to the flicker fusion frequency of normal human vision, say 30/sec. But because the maximum image capture rate of the digital camera is much less than this (of the order of 1 frame/sec), it is not possible to simply grab, transmit, and display a stream of complete high-resolution images as the specimen is scanned under the objective. To overcome this fundamental difficulty without loss of image resolution, the present invention makes use of the fact that during scrolling, between one screen refresh and the next, although the rgb values of every pixel will in general be changed, the bulk of the display if simply shifted slightly in a vertical or horizontal direction so that the information content of the image as a whole remains almost constant.